Heretofore, TFT-LCD which is operated by a thin film transistor (TFT) has been used for a liquid crystal display panel, and a glass substrate for the TFT-LCD is required to have e.g. chemical durability or heat resistance durable against chemicals or heat treatment to is be used in a TFT-forming step. Further, if an alkali metal oxide is contained in the glass substrate, alkali ions in the glass substrate are likely to diffuse into a semiconductor film during the thermal treatment thereby to deteriorate film properties, and therefore the glass substrate is required to contain no alkali metal oxide, namely, to be alkali-free glass.
However, alkali-free glass has a very high viscosity and is difficult to melt, and it has therefore technical difficulties in its production. Further, its production cost tends to be high, and therefore it has been desired to develop a liquid crystal panel employing alkali glass instead of alkali-free glass.
In relation to such development, Patent Document 1 discloses a glass substrate characterized to be used as a glass substrate for a liquid crystal display, made of a glass having a strain point of from 530 to 630° C., a temperature corresponding to 102.5 dPa·s of from 1370 to 1520° C. and a viscosity at a liquid-phase temperature of at least 100,000 poise, wherein the light transmitting surface is a non-polished surface, and the glass contains, as represented by mass %, from 50 to 70% of SiO2, from 1 to 20% of Al2O3, from 0 to 15% of B2O3, from 1 to 25% of alkali metal oxide and from 0 to 30% of alkaline-earth metal oxide. Further, Patent Document 1 discloses a process for producing a glass substrate by forming the glass substrate by a down-draw process.
Further, it is disclosed that even when such an alkali glass substrate is used, if the production temperature (maximum temperature) of TFT is lowered from conventional temperature of about 350 to 400° C., to about 250 to 300° C., it is possible to suppress the diffusion of alkali ions into a semiconductor film, and it is thereby possible to use the alkali glass substrate as a substrate for liquid crystal displays.
If an alkali glass substrate obtained by such a process can be used as an array substrate or an opposing substrate for liquid crystal display panels, it is possible to lower a production cost as compared with a case where alkali-free glass is used, such being preferred.
Further, Patent Document 2 discloses a liquid crystal display device comprising an array substrate having a pixel electrode and a switching active component for operating the pixel electrode formed thereon, and an opposing substrate having an opposing electrode to the above pixel electrode formed on a color filter, wherein a liquid crystal is interposed between the above array substrate and the above opposing substrate, a peripheral area thereof is sealed with a UV-curable sealing material, the thermal expansion coefficient between the above array substrate and the above opposing substrate is different, and its difference is at most 50×10−7/K. Further, Patent Document 2 discloses the above liquid crystal display device employing alkali-free glass for the above array substrate and soda lime glass for the above opposing substrate. Further, it also discloses that soda lime glass is unsuitable for the above array substrate and is used for the above opposing substrate only.
Further, specifically, aluminoborosilicate glass having a thermal expansion coefficient of 38×10−7/K, aluminosilicate glass of 43×10−7/K or bariumborosilicate glass of 46×10−7/K is disclosed as the alkali-free glass, and soda lime glass having a thermal expansion coefficient of 84×10−7/K is disclosed as the soda lime glass. Namely, there is a disclosure of liquid crystal display device in which the difference of the thermal expansion coefficient between the above array substrate and the above opposing substrate is 46×10−7/K, 41×10−7/K or 38×10−7/K. Further, it discloses that it is possible to produce such a liquid crystal display device in such a manner that a UV-curable sealing material is applied on each of the above array substrate and the above opposing substrate, an optimum amount of a liquid crystal is dropped so as to form a specific gap, and both substrates are bonded with each other in vacuum, followed by irradiating them with ultraviolet ray to carry out curing and fixing.
Further, Patent Document 2 discloses that an inexpensive glass substrate is used for such a liquid crystal display device thereby to cut down costs, further sealing and curing can be carried out not by heating but by UV irradiation in a panel production step, and the difference in the thermal expansion coefficient is at most 50×10−7/K, whereby it is possible to prevent warpage of a liquid crystal panel due to heating.
Patent Document 1: JP-A-2006-137631
Patent Document 2: JP-A-2002-350816